The present invention relates to a process for hydrogenating unsaturated hydrocarbons using catalysts comprising copper and zinc. In particular, the invention relates to a process for hydrogenating alkynes using catalysts comprising copper and zinc and especially to a process for hydrogenating alkynes in the presence of alkenes.
In refineries and petrochemical plants, hydrocarbon streams are obtained, stored and processed on a grand scale. In these hydrocarbon streams, unsaturated compounds, whose presence is known to lead to problems especially in processing and/or storage, or which are not the desired product of value, are frequently present, and are therefore undesired components of the corresponding hydrocarbon streams. General overviews of such problems in steamcrackers and typical solutions were given, for example, by H.-M. Allmann, Ch. Herion and P. Polanek in their presentation “Selective Hydrogenations and Purifications in the Steamcracker Downstream Treatment” at the DGMK Conference “Selective Hydrogenation and Dehydrogenation” on Nov. 11 and 12, 1993, in Kassel, Germany, whose manuscript was also published in Conference Report 9305 of the DGMK Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e. V., Hamburg, p. 1-30 (ISSN 0938-068X, ISBN 3-928164-61-9), and M. L. Derrien in: L. Cerveny (ed.), Stud. Surf. Sci. Catal., Vol. 27, p. 613-666, Elsevier, Amsterdam 1986.
Typically, the acetylene secondary component is undesired in C2 streams from steamcrackers, the propyne and allene secondary components are undesired in C3 streams, and the 1- and 2-butyne, 1,2-butadiene and vinylacetylene secondary components are undesired in C4 streams when 1,3-butadiene is to be obtained as the product of value and processed further, and also said secondary components and 1,3-butadiene itself in the cases in which 1-butene, 2-butene (in the cis- and/or the trans form) or isobutene are the desired products. In the processing of C5+ streams (“C5+”: hydrocarbons having at least 5 carbon atoms, “pyrolysis gasoline”), di- and polyenes such as pentadiene and cyclopentadiene, alkynes and/or aromatics with unsaturated substituents such as phenylacetylene and styrene, are undesired when obtaining and processing aromatics or carburetor fuel.
In hydrocarbon streams which stem from an FCC cracker or a reformer instead of a steamcracker, analogous problems occur. A general overview of such problems, especially in C4− and C5+ streams from FCC crackers is given, for example, by J. P. Boitiaux, C. J. Cameron, J. Cosyns, F. Eschard and P. Sarrazin in their presentation “Selective Hydrogenation Catalysts and Processes: Bench to Industrial Scale” at the DGMK Conference “Selective Hydrogenation and Dehydrogenation” on Nov. 11 and 12, 1993, in Kassel, Germany, whose manuscript has also been published in Conference Report 9305 of the DGMK Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e. V., Hamburg, p. 49-57 (ISSN 0938-068X, ISBN 3-928164-61-9).
In general, unsaturated compounds with triple bonds (alkynes) and/or diunsaturated compounds (dienes) and/or other di- or polyunsaturated compounds (polyenes, allenes, alkylenes) and/or aromatic compounds having one or more unsaturated substituents (phenylalkenes and phenylalkynes) should therefore usually be removed from hydrocarbon streams in order to obtain the desired products, such as ethylene, propylene, 1-butene, isobutene, 1,3-butadiene, aromatics or carburetor fuel, in the required quality. Not every unsaturated compound is, however, an undesired component which should be removed from the hydrocarbon stream in question. For example, 1,3-butadiene, as already indicated above, is an undesired secondary component or the desired product of value depending on the case.
The removal of undesired unsaturated compounds from hydrocarbon streams comprising them is frequently done by selectively hydrogenating some or all of the undesired unsaturated compounds in the corresponding hydrocarbon stream, preferably by selective hydrogenation to undisruptive, more highly saturated compounds, and, in a particularly preferred manner, to components of the hydrocarbon streams which constitute the products of value. For example, acetylene is hydrogenated to ethylene in C2 streams, propyne and allene to propylene in C3 streams, butyne to butenes, vinylacetylene to 1,3-butadiene and/or 1,3-butadiene to butenes in C4 streams, and phenylacetylene and styrene to ethylbenzene, cyclopentadiene to cyclopentene and pentadiene to pentene in C5+ streams.
Typically, such compounds should be removed down to residual contents of a few ppm by weight. (“Over-”)hydrogenation to compounds which are more highly saturated than the desired product of value and/or parallel hydrogenation of a product of value comprising one or more multiple bonds to the corresponding more highly saturated or completely saturated compound should, however, be avoided as far as possible owing to the associated loss of value. The selectivity of the hydrogenation of the undesired unsaturated compounds therefore has to be as high as possible. In addition, a sufficiently high activity of the catalyst and a long lifetime are generally desired. At the same time, the catalyst should as far as possible also not bring about any undesired side reactions; for example, catalysis of the isomerization of 1-butene to 2-butenes, with the exception of special cases, should as far as possible be avoided. Processes for selectively hydrogenating unsaturated compounds in hydrocarbon streams comprising them are known both in the form of liquid-phase hydrogenation or mixed gas/liquid phase hydrogenation, in trickle or liquid-phase mode, and also in the form of pure gas phase hydrogenation, various process technology measures for improving the selectivity having been published.
Typically, supported noble metal catalysts in which a noble metal is deposited on a catalyst support are used. Frequently, palladium is used as the noble metal; the support is generally a porous inorganic oxide, for example silica, aluminosilicate, titanium dioxide, zirconium dioxide, zinc aluminate, zinc titanate and/or mixtures of such supports, but aluminum oxide or silicon dioxide are usually used. In addition, promoters or other additives may be present. One disadvantage of noble metal catalysts (“noble metals” in this field of catalysis refer to silver, gold, rhodium, iridium, platinum and palladium) is their relatively high proneness to contaminations, known as “catalyst poisons”, such as mercury, arsenic, sulfur, carbon monoxide and the like. A further disadvantage is the high cost of the noble metals. Although they can generally be recovered from the catalysts, a considerable amount of capital is tied up during their operation. Often, copper-comprising catalysts are therefore also used for hydrogenation, which are considerably more resistant toward catalyst poisons and considerably less expensive.
Copper-comprising catalysts, especially also copper- and zinc-comprising catalysts, are known. They are used predominantly as catalysts, absorbents or adsorbents for the removal of carbon monoxide from gas streams. WO 02/094435 A1 teaches a process for oxidatively removing CO from ethylene at temperatures in the range from 70 to 110° C. over catalysts comprising copper and zinc. U.S. Pat. No. 6,238,640 B1 describes a process for removing carbon monoxide from gas streams comprising hydrogen by reaction with steam and oxygen to give carbon dioxide and hydrogen in the presence of a catalyst which comprises copper oxide and aluminum oxide and at least one metal oxide from the group formed from zinc oxide, chromium oxide and magnesium oxide. DE-A 19 29 977 teaches catalysts comprising from 20 to 60 parts of CuO to 100 parts of ZnO and their use for removing CO from ethylene and propylene streams at a temperature in the range from 50 to 200° C., WO 2004/022223 A2 teaches an adsorption composition comprising copper, zinc, zirconium and optionally aluminum, and its use for removing CO from streams in the completely reduced state.
Catalysts comprising copper and zinc are also known for uses other than for the removal of CO from streams, U.S. Pat. No. 4,593,148 and U.S. Pat. No. 4,871,710 disclose processes for desulfurizing and dearsenating with Cu/Zn catalysts. WO 95/023644 A1 teaches a copper catalyst for the hydrogenation of carbon oxides, for example to methanol, or for the so-called shift reaction of carbon monoxide with water to give carbon dioxide and hydrogen, which, as well as dispersed copper, also comprises stabilizers such as silicon dioxide, aluminum oxide, chromium oxide, magnesium oxide and/or zinc oxide, and optionally also a support such as aluminum oxide, zirconium dioxide, magnesium oxide and/or silicon dioxide. DE 198 48 595 A1 discloses a catalyst for nitrous oxide decomposition of the general formula MxAl2O4 in which M is Cu or a mixture of Cu and Zn and/or Mg, and which may comprise further dopants, especially Zr and/or La. U.S. Pat. No. 4,552,861 teaches a preparation process for catalysts which comprise Cu, Zn, Al and at least one element from the groups formed by the rare earths and zirconium, and also their use for methanol synthesis. The methanol catalysts disclosed in U.S. Pat. No. 4,780,481 comprise Cu, Zn and at least one alkali metal or alkaline earth metal, noble metals and/or rare earths, where Zn may be replaced partly by Zr. U.S. Pat. No. 4,835,132 describes CO shift catalysts which are obtained by calcination from a precursor of the formula (Cu+Zn)6AlxRy(CO3)(x+y)/2OH12+2(x+y)nH2O with layer structure, where R is La, Ce or Zr, x is at least 1 and at most 4, y is at least 0.01 and at most 1.5, and n is about 4.
U.S. Pat. No. 4,323,482 discloses methanization catalysts which comprise chromium and nickel and consist of an intimate mixture of a reducible metal oxide and at least one irreducible metal oxide, and which are activated by reduction at a temperature of from 550 to 1000° C. According to this document, this high temperature leads to fine metals and highly active catalysts. As an aside, mention is also made of the application of this catalyst preparation process to copper-comprising catalysts. U.S. Pat. No. 3,701,739 likewise teaches catalysts composed of a reducible oxide and at least one irreducible oxide, their preparation from an ammoniacal solution of hydroxides or carbonates, and their uses, including for hydrogenation. Mention is made, for instance, of catalysts composed of 30% CuO and 70% ZnO or CuO/ZnO/Al2O3 catalysts. These are used, for example, for the hydrogenation of acetone to isopropanol at 200° C. BE 748 7423 A describes the preparation of catalysts having a series of different active compositions on porous supports by precipitating onto the support with heating, and the use of such catalysts for the hydrogenation of amides at least 50° C. DE-A 20 12 430 discloses conversion catalysts composed of 30-55% by weight of CuO, 25-45% by weight of MgO, 2-30% by weight of Al2O3 and 0-30% by weight of Cr2O3 or ZnO. U.S. Pat. No. 5,990,040 describes conversion catalysts composed of 30-70% by weight of CuO, 20-90% by weight of ZnO, 0.1-20% by weight of an oxide of an element from group IVB, preferably Ti or Zr, 5-50% by weight of Al2O3 and 50-1000 ppm of an oxide of an element from group IA, which may, though, also be used for methanol synthesis, for purification and for hydrogenation. U.S. Pat. No. 6,706,885 B2 teaches a process for preparing 2,5-di(3′-aminoprop-1-ynyl)pyridines by Sonogashira coupling of 2,5-dihalopyridines with protected 3-aminopropynes over copper, zinc or zirconium catalysts.
As already mentioned, the use of copper-comprising catalysts for hydrogenations is also known. WO 96/014280 A1 teaches catalysts which comprise Cu, Zn and at least one compound of Al, Zr, Mg, of a rare earth metal and/or mixtures thereof, and their use for hydrogenating carboxylic esters. EP 434 062 A1 likewise teaches a process for hydrogenating carboxylic esters over a catalyst comprising Cu, Al and a metal selected from the group formed from Mg, Zn, Ti, Zr, Sn, Ni, Co and mixtures thereof. EP 394 842 A1 teaches catalysts comprising 20-75% by weight of NiO, 10-75% by weight of ZrO2 and 5-50% by weight of CuO for the hydrogenation of aliphatic unsaturated compounds such as butynediol at temperatures in the range from 40° C. to 200° C. and pressures of from 30 to 320 bar. EP 646 410 A1 discloses a process for obtaining alcohols by hydrogenation over a catalyst which comprises copper and zinc oxide and a further oxide as the active composition on a support coated with titanium oxide. The hydrogenation process is performed at a temperature of from 160° C. to 350° C. EP 1 331 033 A1 discloses a process for preparing spherical supported metal catalysts by dropletizing a mixture of a polysaccharide and at least one metal compound into a metal salt solution. A CuO catalyst on SiO2 support prepared in this way is used for the hydrogenation of acetophenone at 80° C. and 20 bar of pressure. U.S. Pat. No. 3,677,970 mentions, as well as the sulfur-resistant nickel catalysts for the hydrogenation of hydrocarbons disclosed there, also a series of other catalysts which also include copper catalysts. WO 02/068119 A1 discloses a process for preparing catalysts comprising copper and at least one further element selected from the series of other elements including zinc by size-buildup granulation. These catalysts are used for the hydrogenation of functional organic compounds and for dehydrogenation. WO 2004/026800 A1 describes a process for preparing alcohols by hydrogenating aldehydes over sulfurized copper-zinc oxide catalysts at a temperature of from 240° C. to 280° C. and an (elevated) pressure of from 20 bar to 400 bar.
WO 2004/004901 A1 teaches a process for hydrogenating C4-acetylenes in a liquid hydrocarbon stream over coated catalysts comprising copper on zeolitic support materials at temperatures in the range of from 20 to 80° C. (in the examples, temperatures of 60° C. are used) and pressures of 15 and 50 bar. N. L. Carr, D. L. Stahlfeld and H. G. Robertson, in Hydrocarbon Processing, May 1985, p. 100-102, report copper-comprising absorption compositions for removing arsenic from olefin streams. The hydrogenation of the olefins is in this case a side reaction which can be suppressed by avoiding temperatures above of 250° F. (corresponding to 121° C.). J. Blanco, in Quimica e Industria 20 (1974) 604-606, reports that hydrocarbons are hydrogenated over copper catalysts only at temperatures of at least 300° C. and pressures around 300 bar.
One disadvantage of common hydrogenation catalysts with copper as the hydrogenation-active metal is accordingly that relatively high hydrogenation temperatures are needed. However, some streams already exhibit decomposition phenomena at these temperatures, for example, in typical propylene streams—which always also comprise traces of oxygen—oxygenates are formed even from 50° C. Such oxygenates can act as catalyst poisons in downstream processes, for instance preparation of polypropylene over metallocene catalysts, and are therefore extremely undesirable.
The requirements on processes for the selective hydrogenation of undesired unsaturated compounds are rising constantly. It is therefore an object of the invention to find an improved process for selectively hydrogenating unsaturated compounds, especially a process which avoids the formation of by-products such as oxygenates. At the same time, activity and selectivity of the catalysts should be high.